As the complexity of communication systems increases, there is more and more need for additional ways of transmitting information. One method of communication is called Asymmetrical Digital Subscriber Line (ADSL) and is a transmission scheme that allows for the provision of plain old telephone service (POTS) and a variety of digital channels on two/wire twisted metallic wire pair with mixed gauges. It is desirable to use such twisted wire as there is an existing infrastructure, the use of which will reduce installation costs. ADSL standards are proposed in the draft American National Standard for Telecommunication--Network and Customer Interfaces--Asymmetric Digital Subscriber Line (ADSL) metallic interface, T1, E1.4/95-007R2, ADSL coding standard, draft, Aug. 15, 1995.
One of the advantages of ADSL transmission is that it may be used to provide high quality, multiple and simultaneous interactive video services. This is possible communicating over an ordinary telephone line without disruption of the standard telephone service. In traditional data communication systems, the transmission of clocking information is embedded in the data stream and may be implemented in a variety of ways, depending on line code technique. However, according to the ADSL standards, no such clocking information is permitted to be included in the transmission of ADSL data.
The prohibition of including clocking information in the transmission flow presents a problem in an ADSL system where an external network may provide input data streams driven at varying frequency which are independent of each other, and which are additionally independent of the ADSL transmission system clock. Under such conditions channel synchronization will be required between transmitting and receiving transceivers. Here the transmitter is referred to as a far end transceiver and the receiver is referred to as a near end transceiver.
In order to synchronize between far end and near end transceivers, the near end transceiver will require some method of tracking the far end data clock and adjust its data clock accordingly. Typically, a digital phase lock loop (DPLL) will be used for such a purpose.
According to ADSL standards, all input ports must be designed to tolerate a digital signal having electrical characteristics in accordance with the requirements of recommendation G.703. Additionally, recommendations G.823 and G.824 define further clock requirements regarding sinusoidal wander and jitter effects. Typically, wander effects are considered to be predominant at frequencies below 20 hertz, and for the most part, will influence the size of an input port first in first out (FIFO) logic.
To further satisfy ADSL standards, it is necessary to accommodate an ADSL byte stuff/delete operation performed at the far end, which causes a data arrival rate variation from the expected data arrival rate. This requires that a near end DPLL be a function of the expected data arrival rate adjusted by a far end byte stuff or byte delete operation due to far end data clock phase variations.
The ADSL standards specify transmission of data at frequencies equal to or above 32 kHz. This leaves the problem of transmitting 16 kHz control channel information on an ADSL system.
To solve these problems, it is desirable to have a near end transceiver capable of synchronization with a far end transceiver, which will be able to independently track the far end data clock and adjust its clock accordingly. Obviously, to operate within the ADSL domain requires conformity to regulations referred to in the ADSL standards mentioned here above.